Aetiology of Malocclusion I N Flashcards
what are the 3 general aetiological factors in the formation of a malocclusion?
- Skeletal issues
- Size of the jaws
- Shape and relative positions of the jaws - Muscular issues
- Form and function of the muscles surrounding the teeth - Dentoalveolar issues
- the size of the teeth in relation to the size of the jaws
what does malocclusion result from in the facial skeleton?
disharmony in the components of the facial skeleton
what are the components of the facial skeleton we consider involved in the formation of a malocclusion?
- the maxilla
- the mandible
- the alveolar processes
- the cranial base angle
what does the transverse view of the face look at?
the alignment of the jaws and how they meet
what genetic factors play a role in skeletal variation?
familial studies, particularly those with twins, show there is a genetic component the shape of the jaws and how they relate
particularly in class 3
what environmental factors can play a role in skeletal variation?
- masticatory muscles
- mouth breathing
- head posture (up and forward posture = longer facial pattern) (back and downward posture = deep overbite and reduced face height)
if a patient has weak masticatory muscles, how are they likely to appear?
weaker muscles tend to have longer faces and are prone to an anterior open bite
if a patient has strong masticatory muscles, how are they likely to appear?
reduced lower anterior face height and deep bite
how can a patient have abnormal jaws but still be a class 1?
they both could be over or undersized, but still relate normally to one another
what is the class 1 skeletal relationship?
the mandible and maxilla relate normally to one another, with the mandible 2-3mm posterior to the maxilla
what is lateral cephalometry?
this is a standardised lateral radiograph of the face and the skull
o it is standardised so that it is reproducible if the patient is positioned in a cephalostat a set distance from the cone and film
what is the SNA measurement?
the angle created by the sella, nasion and A point
what is the SNB measurement?
the angle created by the sella, nasion and B point
what is the ANB measurement?
the angle created by the A, B and N points
what are the average cephalometric values for a class 1 diagnosis?
SNA = 81+/-3
SNB = 78+/-3
ANB = 3+/-2
what are the potential causes of a class 2 skeletal pattern?
- The mandible being too small (most common)
- The maxilla being too large
- The mandible being normal sized but places too far back due to an obtuse cranial base angle
how will the teeth erupt if a patient has a class 2 skeletal relationship?
in a post normal class 2 buccal segment relationship
what are the average cephalometric values for a class 2 diagnosis?
SNA = usually average but may be increased if the maxilla is prognathic
SNB = <78+/-3 (usually decreased)
ANB = >4 (sometimes >5)
when is the Eastman correction used and what is it?
- Correct the ANB for a half degree for every degree SNA is over or under the average
- For every degree OVER 81 -0.5o, and for every degree UNDER 81 +0.5o to ANB
used for when the SNA is larger or smaller than usual
what are the potential causes of a class 3 skeletal pattern?
- Maxilla being too small (most commonly)
- the mandible is too large
- a combination of both
- normally sized jaws but the mandible is positioned too far forwards due to an acute cranial base angle
how do the teeth erupt in a class 3 skeletal relationship?
in pre normal class 3 buccal segment relationship
what are the average cephalometric values for a class 3 diagnosis?
SNA = sometimes decreased if the maxilla is retrognathic/hypoplastic
SNB = often average as usually the maxilla is too small but may be increased if the mandible is prognathic
ANB = <1 or negative
what is dents alveolar compensation?
o the dento-alveolar structures may disguise an underlying skeletal discrepancy
o forces from the lips,cheeks and tongue tend to incline the teeth toward a position of soft tissue balance
e.g. in a class 3, the incisors erupt more vertically to meet anteriorly but this leads to an increased maxillary height
what might happen in dent alveolar compensation if there is a reduced cranial base angle?
the posterior teeth may erupt more vertically so that there is an anterior open bite
how do you assess the vertical jaw relationship?
by looking at the FMPA (angle between the Frankfort plane and the mandibular plane)
what does the average clinical value of the vertical jaw relationship measure?
the upper anterior face height (from glabella to the base of nose) and the lower anterior face height (from the base of the nose to the tip of the inferior aspect of the chin)
if done visually, what is the average ratio of the lower anterior face height to the total anterior face height?
50%
what is the average cephalometric value of the FAMP in the vertical skeletal relationship?
27 degrees +/-4
what is the average LAFT?TAFH ratio in cephalometric analysis?
55%
what points are used to measure the UAFH and LAFH in cephalometry?
UAFH = nasion to the anterior nasal spine
LAFH = anterior nasal spine to the mention
in a long facial type, what will the FAMP and LAFH/TAFH values be?
LAFH/TAFH = >55% (i.e. LAFY is greater)
FAMP = >31 degrees
in a short facial type, what will the FAMP and LAFH/TAFH values be?
LAFH/TAFH = <55%
FAMP = <23 degrees
how will a patients face appear in a long anterior face?
- A steeply inclined mandibular plane
- Backward mandibular growth rotation
- A tendency for an anterior open bite
how will the patients face appear in a short facial type?
- Shallower mandibular plane
- Results in a more defined chin point
- The jaws are almost parallel
- There is a forward mandibular growth rotation
- May increase depth of bite over time - Tendency to have a deep overbite
what are arch width discrepancies?
discrepancies in the correct relative width of the jaws in relation to one another
e.g. the maxilla should always be wider than the mandible
what can exaggerate arch width discrepancies?
AP skeletal discrepancies
what can result from arch width discrepancies?
mandibular displacement, bilateral or unilateral buccal segment corssbites and transverse dentoalveolar compensation
how would you assess a patients mandibular displacement?
look at their teeth in the initial tooth contact, and then get them to slide into intercuspation and observe if there is deviation of the mandible
what is mandibular displacement commonly associated with?
TMJ disorders
if a patient cannot reach a centric occlusion in ICP or RCP, what might they have?
a skeletal asymmetry
why is a mandibular displacement common in class 3 occlusions?
the maxilla bites onto a wider part of the mandible as it is set further back resulting in a lack of intercuspation
what is transverse dento alveolar compensation?
this is compensation in the transverse plane where the tongue and cheeks force the teeth to erupt on a different inclination to prevent a crossbite
how might there be transverse dento alveolar compensation if there is a narrow maxilla?
the maxillary molars erupt more flared
how might there be transverse dents alveolar compensation if there is a wide maxilla?
the lower molars erupt more upright
what are the 2 causes of facial asymmetries?
- dental causes
- displacement of the normal mandible due to a unilateral crossbite - true mandibular asymmetry
- hemi-mandibular hyperplasia/elongation
- condylar hyperplasia
the whole face may be affected by mild expressions of hemi-facial microsomia
how can an arch size discrepancy cause crowding?
small jaws and normal size teeth
what are the 2 methods of crowding?
- small jaws and normally sized teeth OR
- normal sized jaws and large teeth (macrodontia)
what are the 2 methods of spacing?
- large jaws and normally sized teeth
- normal sized jaws and small teeth (microdontia
